Weld quality monitoring in friction stir lap joining of dissimilar Al6061 to polycarbonate using thrust–torque-based process stability signatures

The joining of dissimilar aluminium-based alloys to thermoplastic polymers is highly challenging which indicates the necessity of friction welding procedures. The present work addresses the degrees of steadiness in welding thrust-torque to monitor the lap weld contour of aluminium alloy (Al6061) overlapped on polycarbonate sheet or vice versa using friction stir welding. The primary objective was to compare the monitoring capability of the weld non-uniformity by using raw signals and stability signature of welding thrust and stirring torque. Further, these stability indices were used for the prediction of weld mechanical behaviour using response surface methodology. The stability signatures were found to be a better indicator of weld profile unevenness than raw signals as it was an amplified version of the local deviations in welding thrust-torque. Improved strength efficiency up to 49% was noticed in PC-Al welds due to improved process stability at intermediate tool traverse (55 mm/min) and revolving speeds (1100 rpm). The stability in stirring torque was better indicator of weld quality features than tool thrust steadiness in Al-PC lap while it was reversed in PC-Al lap. Thus, the thrust force instability was mostly indicated the joint ductility while combined steadiness in thrust-torque were essential for the prediction of weld strength. The higher stability index in tool thrust (≥7.5) with ample torque steadiness (5–8) can upgrade the Al-PC weld strength (≥20 MPa), whereas intermediate thrust stability (6–6.5) with high stable torque (≥7) can uplift the joint strength (≥25 MPa) in PC-Al welds.